River rock is a creative method of decorating a floor or (more often) a wall, using mounded, natural rocks set in mortar and grout. Today, you can get ceramic representations of river rock, which are easier to lay out and cut but have the same rustic, striking look -- and the same complications when it comes to grouting them. The irregular shape of the tiles, and the varying size of the lines between them, pose a challenge even if you've had experience grouting tile before.
Install your river rock ceramic tile as instructed, on a bed of cement board for stabilility. The tile is secured with thinset mortar spread with a notched trowel, and irregular spaces are left between the tiles, anywhere from 1/4 inch to 1 inch. As you lay the tiles, make sure you don't have mortar bunching up in the lines, as it will dry like that and interfere with the grouting; use the edge of a small putty knife to scoop out any bunched-up mortar from the lines while it's still wet.
Grout is a sand-mortar mixture about the consistency of cake frosting. Normally, it's spread over tiles with your flat rubber grout float so that it settles into the spaces between the tiles, then you wipe it off the tile face. This won't work with river rock, because the spaces are too varying in size and the tile face too uneven. Instead, scoop the grout out of the bucket with a plastic putty knife and set it directly into the lines, letting it mound up. Put on rubber glovers and use your fingertips to press it into the lines and smooth out the top. Use sponge to wipe the excess off the tops of the tiles, taking care not to disturb the grout you just laid.
Sealing the grout with a clear chemical grout sealer protects it from moisture and staining. Allow the grout to set for two or three days first. Apply the sealer with a small brush, making sure to completely cover the grout lines, while keeping it off the tiles. Have a damp cloth handy to wipe it off the tiles of it gets on them. Allow the first coat of sealer to dry for a day, then apply a second coat of sealer in the same manner.
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